The present invention relates generally to the field of implantable leads and pacing systems for monitoring and controlling the heart. More specifically, the present invention is directed to a myocardial tissue stimulating electrode and lead which interconnects to a pacing system, the myocardial stimulating electrode being particularly adapted for implant and use in infants and small children.
There are two general approaches to implanting a pacing lead in a recipient of a pacing system. The first approach utilizes a pacing lead inserted transvenously, which extends into an abuts an interior endocardial surface of the wall of the heart. Alternatively, pacing systems may include an epicardial lead with an electrode which is affixed in the myocardial tissue of the heart, interconnected via the epicardial lead to the pacemaker. Epicardial pacing is often the required approach for use with pediatric patients because of congenital or surgically induced anatomical considerations and/or due to the size of the pacemaker itself, the size of a transvenous pacing lead as compared to the veins of the pediatric patient, or problems associated with the pacing leads interfering with the tricuspid valve.
However, there are problems associated with epicardial pacing relating to the design of the epicardial lead, including failure of the electrode and resultant pacing threshold "exit block," as well as the limitations on the expected operative life of the epicardial leads and electrodes. It should be noted that there are three basic types of electrode designs for epicardial leads. The first design is referred to as a "sutured lead" and typically includes either a "button,""patch," or "needle-like" electrode which is sutured onto the epicardial surface, or into the myocardium by passing the suture through the base of the electrode and into the adjacent epicardial tissue.
A second type of epicardial lead is a "sutureless" lead, which includes a fish hook or barbed design electrode formed from a metallic element having sufficient structural capability to allow insertion and self-retention in the epicardial tissue. A third type of epicardial lead is also a "sutureless" type electrode and is a screw-in electrode which incorporates an electrode formed into a cork screw configuration adapted to be screwed into the epicardial tissue. These types of screw-in electrode designs may include two or three convolutions, having various sizes and pitch lengths.
For all of the above types of electrodes, there are several different failures which may occur. The first type of failure is associated with an elevated threshold and a high electrode impedance. This type of failure may also be termed as an "exit block" failure. Exit block occurs when a functioning cardiac pacemaker and intact lead system are unable to transmit sufficient energy to the myocardium for consistent cardiac pacing. A second type of failure is associated with electrodes that will not pace at any output of the pacing system. This may occur, for example, because of the failure of the electrode, electrode evulsion, or fracture of the wire interconnecting the pacing generator and the electrode. A third type of failure consists of electrodes having a high threshold but a low electrode impedance. This type of failure may be due to either an insulation fracture or erosion of the stimulating portion of the electrode into the ventricular cavity.
A number of studies have addressed the problems associated with lead failures and pacing systems especially in pediatric patients for various types of epicardial lead electrode configurations. The studies have found that one of the major problems with these types of systems is the high incidence of failure. One study found that lead failures, caused for example by electrode fracture, occurred on an average of twenty eight months after implant. As a result, infants receiving an epicardial electrode pacing system almost always require subsequent surgery within several years and frequently within the first two years. Due to the limitations of the design and failure rate of the epicardial lead, it is extremely unlikely that a pacing system using an epicardial lead of a current design will last until the patient reaches adulthood.
In view of the foregoing limitations of presently available epicardial electrodes and pacing systems, an alternative design for the epicardial electrode lead which provides the capability of enhanced reliability and extended life expectancy is very desirable.